Cool thread by the way.
Probably air bending, since i'm a bit of a pacifist, and airbending seems to be the one that is least lethal. Not to mention, the diverse capabilities of airbending, the possibilities are potentially endless.
Plus its the closest thing to the force the Avatar universe has to offer.
I don't know how the TV show portrays it, but realistically if you can control Air, you should also be able to generate lightning and heat via wind friction. You could also have limited control over bodies of water and be able to generate storms. And you'd be able to control less-dense earthen substances such as sand and dirt via the wind as well.
For all intends and purposes (again, not clear on Avatar's "rules") assuming your control and power was great enough, Air-bending is basically a more unrefined form of telekinesis.
Also...flying. 'Nuff said.